As Oprah moments go, the story about a genealogist who reunites with her long-lost birth father is about as heartwarming as they come.

Just one problem — the “dad” says it isn’t true.

That didn’t stop Pamela Slaton from featuring the reunion in a book that led to a show on Oprah Winfrey’s cable network OWN.

And now Vincent Oleniak, a retired mortician from The Bronx, is suing, saying that he “never acknowledged paternity” and that the author “has knowingly, intentionally and falsely identified [him] as her biological father in the book.”

That’s not all. According to the suit, filed yesterday in Manhattan Supreme Court, Chapter Eight of Slaton’s “Reunited: An Investigative Genealogist Unlocks Some of Life’s Greatest Family Mysteries” trashes Oleniak as a mob-connected ladies man who hits on his own daughter and abandoned her because he’s sexist.

“We may not be living out the father-daughter fairy tale, but it matters to me that he exists,” she writes in the nonfiction tome.

The suit also names Slaton’s publisher, Macmillan, and her ghost writer, Samantha Marshall.

As for Winfrey and her network, they are not named in the suit, but a spokeswoman said Slaton’s show ran for three months in 2011 on the cable channel, but was canceled because of low ratings, and not anything connected to the book.

Oleniak, who declined to comment, is seeking unspecified compensatory and punitive damages.

The suit said Slaton made no effort to hide Oleniak’s identity in the book.

He admits that he knew the author when she was a child, but said his relationship with Slaton’s mother started after the mother was already pregnant with Slaton.